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authorValentin Popov <valentin@popov.link>2024-07-19 15:37:58 +0300
committerValentin Popov <valentin@popov.link>2024-07-19 15:37:58 +0300
commita990de90fe41456a23e58bd087d2f107d321f3a1 (patch)
tree15afc392522a9e85dc3332235e311b7d39352ea9 /vendor/adler/src/algo.rs
parent3d48cd3f81164bbfc1a755dc1d4a9a02f98c8ddd (diff)
downloadfparkan-a990de90fe41456a23e58bd087d2f107d321f3a1.tar.xz
fparkan-a990de90fe41456a23e58bd087d2f107d321f3a1.zip
Deleted vendor folder
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-rw-r--r--vendor/adler/src/algo.rs146
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diff --git a/vendor/adler/src/algo.rs b/vendor/adler/src/algo.rs
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-use crate::Adler32;
-use std::ops::{AddAssign, MulAssign, RemAssign};
-
-impl Adler32 {
- pub(crate) fn compute(&mut self, bytes: &[u8]) {
- // The basic algorithm is, for every byte:
- // a = (a + byte) % MOD
- // b = (b + a) % MOD
- // where MOD = 65521.
- //
- // For efficiency, we can defer the `% MOD` operations as long as neither a nor b overflows:
- // - Between calls to `write`, we ensure that a and b are always in range 0..MOD.
- // - We use 32-bit arithmetic in this function.
- // - Therefore, a and b must not increase by more than 2^32-MOD without performing a `% MOD`
- // operation.
- //
- // According to Wikipedia, b is calculated as follows for non-incremental checksumming:
- // b = n×D1 + (n−1)×D2 + (n−2)×D3 + ... + Dn + n*1 (mod 65521)
- // Where n is the number of bytes and Di is the i-th Byte. We need to change this to account
- // for the previous values of a and b, as well as treat every input Byte as being 255:
- // b_inc = n×255 + (n-1)×255 + ... + 255 + n*65520
- // Or in other words:
- // b_inc = n*65520 + n(n+1)/2*255
- // The max chunk size is thus the largest value of n so that b_inc <= 2^32-65521.
- // 2^32-65521 = n*65520 + n(n+1)/2*255
- // Plugging this into an equation solver since I can't math gives n = 5552.18..., so 5552.
- //
- // On top of the optimization outlined above, the algorithm can also be parallelized with a
- // bit more work:
- //
- // Note that b is a linear combination of a vector of input bytes (D1, ..., Dn).
- //
- // If we fix some value k<N and rewrite indices 1, ..., N as
- //
- // 1_1, 1_2, ..., 1_k, 2_1, ..., 2_k, ..., (N/k)_k,
- //
- // then we can express a and b in terms of sums of smaller sequences kb and ka:
- //
- // ka(j) := D1_j + D2_j + ... + D(N/k)_j where j <= k
- // kb(j) := (N/k)*D1_j + (N/k-1)*D2_j + ... + D(N/k)_j where j <= k
- //
- // a = ka(1) + ka(2) + ... + ka(k) + 1
- // b = k*(kb(1) + kb(2) + ... + kb(k)) - 1*ka(2) - ... - (k-1)*ka(k) + N
- //
- // We use this insight to unroll the main loop and process k=4 bytes at a time.
- // The resulting code is highly amenable to SIMD acceleration, although the immediate speedups
- // stem from increased pipeline parallelism rather than auto-vectorization.
- //
- // This technique is described in-depth (here:)[https://software.intel.com/content/www/us/\
- // en/develop/articles/fast-computation-of-fletcher-checksums.html]
-
- const MOD: u32 = 65521;
- const CHUNK_SIZE: usize = 5552 * 4;
-
- let mut a = u32::from(self.a);
- let mut b = u32::from(self.b);
- let mut a_vec = U32X4([0; 4]);
- let mut b_vec = a_vec;
-
- let (bytes, remainder) = bytes.split_at(bytes.len() - bytes.len() % 4);
-
- // iterate over 4 bytes at a time
- let chunk_iter = bytes.chunks_exact(CHUNK_SIZE);
- let remainder_chunk = chunk_iter.remainder();
- for chunk in chunk_iter {
- for byte_vec in chunk.chunks_exact(4) {
- let val = U32X4::from(byte_vec);
- a_vec += val;
- b_vec += a_vec;
- }
- b += CHUNK_SIZE as u32 * a;
- a_vec %= MOD;
- b_vec %= MOD;
- b %= MOD;
- }
- // special-case the final chunk because it may be shorter than the rest
- for byte_vec in remainder_chunk.chunks_exact(4) {
- let val = U32X4::from(byte_vec);
- a_vec += val;
- b_vec += a_vec;
- }
- b += remainder_chunk.len() as u32 * a;
- a_vec %= MOD;
- b_vec %= MOD;
- b %= MOD;
-
- // combine the sub-sum results into the main sum
- b_vec *= 4;
- b_vec.0[1] += MOD - a_vec.0[1];
- b_vec.0[2] += (MOD - a_vec.0[2]) * 2;
- b_vec.0[3] += (MOD - a_vec.0[3]) * 3;
- for &av in a_vec.0.iter() {
- a += av;
- }
- for &bv in b_vec.0.iter() {
- b += bv;
- }
-
- // iterate over the remaining few bytes in serial
- for &byte in remainder.iter() {
- a += u32::from(byte);
- b += a;
- }
-
- self.a = (a % MOD) as u16;
- self.b = (b % MOD) as u16;
- }
-}
-
-#[derive(Copy, Clone)]
-struct U32X4([u32; 4]);
-
-impl U32X4 {
- fn from(bytes: &[u8]) -> Self {
- U32X4([
- u32::from(bytes[0]),
- u32::from(bytes[1]),
- u32::from(bytes[2]),
- u32::from(bytes[3]),
- ])
- }
-}
-
-impl AddAssign<Self> for U32X4 {
- fn add_assign(&mut self, other: Self) {
- for (s, o) in self.0.iter_mut().zip(other.0.iter()) {
- *s += o;
- }
- }
-}
-
-impl RemAssign<u32> for U32X4 {
- fn rem_assign(&mut self, quotient: u32) {
- for s in self.0.iter_mut() {
- *s %= quotient;
- }
- }
-}
-
-impl MulAssign<u32> for U32X4 {
- fn mul_assign(&mut self, rhs: u32) {
- for s in self.0.iter_mut() {
- *s *= rhs;
- }
- }
-}